Orbits of the two STEREO spacecraft

When the two STEREO spacecraft are launched, they are first placed in
a highly eccentric orbit around Earth. In this Quicktime movie,
the green dot represents Earth, the white dot stands for the Moon, and the
letters "A" (red) and "B" (blue) stand for the STEREO "Ahead" and "Behind"
spacecraft respectively. The yellow arrow represents the direction of the Sun,
which changes as Earth moves along its orbit.

For the first few weeks, the two spacecraft are drifting slowly apart, but
still stay fairly close to each other as they line up for their close encounter
with the Moon one month into the mission. As they fly by, the Moon's gravity
flings the two spacecraft out away from Earth. This is when the relatively
small distance between the two spacecraft makes all the difference.
The "Ahead" spacecraft is flung completely away from Earth, and becomes a
satellite of the Sun, while the "Behind" spacecraft curves back to fly past the
Moon a second time six weeks later, and gets flung out in the opposite
direction.

This second Quicktime movie
shows the two spacecraft once they've both settled into orbits around the Sun.
As in the first movie, the green dot represents Earth, and the red "A" and
blue "B" represent the STEREO "Ahead" and "Behind" spacecraft. The yellow
dot represents the Sun. The orbits of Mercury, Venus, and Mars are also
shown. The orbits of the two STEREO spacecraft are both more eccentric than
Earth's, with the "Ahead" spacecraft orbiting slightly inside Earth's orbit,
and the "Behind" spacecraft orbiting slightly outside. Because of these slight
differences in the average distance from the Sun, the two spacecraft slowly
drift away from Earth in opposite directions.